“It was because of [Washington] that we did the play. Because [the studio] was perfectly willing to go ahead and do the film, but he said he needed to do the play first,” Henderson, 67, tells The Post.

“Somewhere in the interim, [Washington] decided he wanted to direct it,” he says. “He would always first take care of every other actor in any scene — and then turn the camera on himself.”

The movie, already an Oscar favorite, reunites the cast of the Tony-winning 2010 revival. Henderson and Davis, who went on to star in “The Help” and ABC’s “How To Get Away With Murder,” first appeared together in Wilson’s “King Hedley II” 15 years ago.

“As the public got the chance to see her in more demanding and highly visible roles, it appears that they’ve seen her grow as an artist,” Henderson says of the actress. “But she was fully grown when I first saw her, boy, I tell ya!”

Denzel Washington and Stephen McKinley Henderson in a scene from “Fences.”Paramount Pictures

Wilson’s plays have formed the backbone of Henderson’s career. Since he saw his first, “Joe Turner’s Come and Gone,” in 1988, he’s performed in eight of the Pulitzer winner’s 10 “Pittsburgh Cycle” plays, receiving a Tony nod for “Fences” and touring the world in “Jitney.”

He met the late playwright in 1990, after driving to Pittsburgh to hear Wilson give a talk. But the local newspaper botched the start time, and Henderson arrived two hours late.

“ ‘Oh, my God, I have to go back to Buffalo,’” Henderson remembers saying aloud. “Then I hear, ‘You live in Buffalo?’ And [a woman] asked me if I knew this actor, Cashmere Ellis.”

“I said, ‘Yeah, I know Cashmere.’ ”

“She said, ‘Well, if you take him a message from me, I’ll introduce you to my brother.’ ”

The woman was Wilson’s sister.

Pittsburgh was the playwright’s hometown and the setting for most of his works. Washington filmed the movie there to give “Fences” a more authentic feel.

“To film it in Pittsburgh, and have Pittsburgh citizens who are walking the streets in the Hill District,” Henderson says. “I know how much August would’ve loved that.”